From: Eastern Nebula
I would say that just adding more resources... doesn't add anything, neither in terms of gameplay (at best, you just redistribute tensions, at worst, you remove some tensions for resource control) nor immersion (the more you have resources, the less significance each of them has).
What would a tiered economy bring? Layers of strategic complexity, an expansion of the research tree and definitely more immersion. Not only you need to protect your sources and transit lanes, but you have to make sure your distribution between raw and manufactured is efficient, and, conversely, targeting different stages of resource production when waging war can have very different effects on how it cripples the enemy. You also expand the whole aspect of quantity vs. quality, much more than with just raw resources of different values.
Some strategy games have tried that with more or less success (Anno, Stellaris to an extent) for these reasons.
All that being said, I'm leaning more and more towards an expansion of the ground combat mechanics + colony building and management... Of course, it should be balanced, not create micromanagement sinkholes.